Photosynthesis is the conversion of light energy to chemical energy by biological systems. The first step of photosynthesis is the absorption of light by pigment-protein complexes. These complexes channel light energy to the photosynthetic reaction center, where light energy excites electrons that are transferred from pigment molecules through an electron transport chain that harvests the energy for biochemical reactions.
Microalgae, such as cyanobacteria, can be cultured photosynthetically for the production of various products, including proteins, peptides, amino acids, carbohydrates, isotopically labeled compounds, terpenoids, carotenoids, pigments, vitamins, and lipids, where light provides the energy for growth and biosynthesis of the algal products. Microalgal production systems can utilize open ponds (Ben-Amotz (1995) J. Appl Phycol 7: 65-68; Olaizola (2000) J. Appl Phycol 12: 499-506) or photobioreactors (Olaizola (2000) J. Appl Phycol 12: 499-506; Xu et al. (2009) Eng. Life Sci 9: 178-189; Lehr and Posten (2009) Curr Opinion Biotechnol 20: 280-285; US2009/0011492; WO2011/143619) where the energy for growth and production may be provided by natural or artificial light. In order to minimize production costs and maximize volumetric yield of photosynthetic microorganisms, it is desirable for the photosynthetic microorganisms to be grown in large volumes that reach high cell density. However, light penetration of an algal culture declines dramatically as culture depth and cell density increase. Active mixing of algal cultures propagated in a pond or photobioreactor allows the cultured algal cells to be exposed to higher levels of light intermittently when they are in proximity to the surface or light-facing boundary of a pond or photobioreactor. Actively mixed cultures cells experience some time periods of sub-optimal light, as well as some periods when the cells are at or close to the surface or perimeter of a pond or bioreactor where light may be super-saturating.
Algae typically use only a percentage of the solar radiation incident on a pond surface, and photosynthesis can be inhibited by excess solar radiation. When photosynthetic microorganisms are exposed to light of an intensity that is greater than the capacity for photosynthetic utilization, as may occur at the upper level of a pond or the periphery of a photobioreactor culture, the photosynthetic microorganisms may engage mechanisms for light energy dissipation to limit damage to the photosynthetic apparatus that might otherwise be caused by absorption of excess light energy.
Light energy can be lost from the pigment-protein complexes through mechanisms including fluorescence or by cell-regulated processes such as Non-Photochemical Quenching (NPQ). The qE component of NPQ is a protective mechanism that quenches singlet-excited chlorophylls (Chl) and harmlessly dissipates excess excitation energy as heat. These NPQ processes help to regulate and protect the photosynthetic apparatus from damage in environments in which light energy absorption exceeds the capacity for light utilization. In the absence of intrinsic NPQ mechanisms, such as energy dissipation mediated by carotenoids, photosynthetic organisms can incur photooxidative damage under water or nutrient limitation, low temperatures, and/or high light intensity (Demmig-Adams et al. (1996) FASEB J. 10: 403-412). In many cyanobacterial species, the Orange Carotenoid Protein binds the carotenoids zeaxanthin, echinenone, and/or hydroxyechinenone, and serves a photoprotective function in these species. Synechocystis cells having a mutant OCP gene had a greater decrease in photosynthetic activity than corresponding wild type cells in response to high light intensity (Wilson et al. The Plant Cell (2006) 18: 992-1007, and cyanobacterial species lacking an OCP gene were more photosynthetically impaired under high light conditions than species that have an OCP gene (Boulay et al. (2008) Biochimica et Biophysica Acta 1777: 1344-1354).